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Trang 1Improved Wind Power Conversion System
Using Magnetic Energy Recovery Switch (MERS)
Taku Takaku∗, Gen Homma∗, Takanori Isobe∗, Seiki Igarashi†, Yoshiyuki Uchida† Ryuichi Shimada∗
∗Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology
N1-33, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan Telephone: +81-3-5734-3328, Fax: +81-3-5734-2959
E-mail: ttakaku@nr.titech.ac.jp
†Fuji Electric Device Technology Co., Ltd.
Gate City Ohsaki, East Tower, 11-2, Osaki 1-chome, Shinagawa-ku, Tokyo, 141-0032, Japan Telephone: +81-3-5435-7153, Fax: +81-3-5435-7466 E-mail: igarashi-seiki@fujielectric.co.jp
Abstract— This paper presents experimental results on an
innovative power conversion technology using magnetic energy
recovery switch (MERS) of a wind turbine system with a
synchronous generator to improve the output power and the
efficiency An output voltage of the synchronous generator
decreases with the increase of current because of synchronous
reactance The MERS, which consists of four MOSFET or IGBT
elements and one small DC capacitor just like as a full bridge
configuration, is inserted in series between the generator and
ac-dc converter The capacitor absorbs the magnetic energy stored
in the synchronous inductance by forced LC resonance Since
MERS compensates the reactance voltage of the synchronous
generator by the capacitor voltage, the output voltage of the
generator increases and the excitation current of the generator
can be extremely reduced Also, the switching loss of converter in
the MERS system is very small because the MERS is not required
PWM control and the switching frequency is the same as the
generator, and the downsizing of converter is realized The effect
of MERS is verified in a small scale experimental set-up of wind
power generation with a permanent magnet type synchronous
generator and a dc-excitation type synchronous generator The
data indicate a great potential of the new power conversion
technology to make the actual wind turbine system compact and
to improve the efficiency.
Wind turbine system is rapidly developing as one of the
most promising renewable energy resources over the world
The penetration of this system is very important to solve
the global warming and the exhaustion of fossil fuel In
order to optimize the wind turbine system, many kinds of
power conversion systems to connect between the generator
and the grid line have been proposed and utilized The wind
power is always fluctuating, and the maximum power far
exceeds the ratings value, and it often reaches 1.5 times the
ratings value Therefore, a new electric power conversion
technology different from a conventional power generation
technology is necessary for the wind power generation system
We do believe that there is still a room to improve the power
conversion system to obtain more economical systems
We have proposed a bi-directional magnetic recovery switch
(MERS)[1] The MERS is a quite useful switch having a
S 3
S 4
P o w e r S o u r c e
M O S F E T
o r I G B T
switch (MERS) The MERS is inserted in series between power source and load.
bridge configuration with four IGBTs or MOSFETs and a small capacitor A power factor correction is possible regard-less of the impedance and power frequency of the load by the automatic synchronized switching[2]
We are intending to apply the MERS to the power con-version of wind turbine system for improving the system performance As the first step of this project this paper presents, for the first time, a wonderful effect of MERS in a small scale experimental set-up of wind power generation with
a permanent magnet type and a dc-exciting type synchronous generator
II MAGNETICENERGYRECOVERYSWITCH(MERS) The basic configuration of MERS is shown in Figure 1 Four IGBTs (or MOSFETs) are connected in two parallel arms Each arm consists of two IGBTs connected in series Four
Trang 2S G
C o n v e r t e r I n v e r t e r G r i d L i n e
W i n d T u r b i n e
G e n e r a t o r
(a) Conventional back-to-back system.
M E R S
S G
D i o d e
B r i d g e I n v e r t e r
G r i d
L i n e
W i n d T u r b i n e
G e n e r a t o r
(b) Newly proposed MERS system 3-phase MERS is inserted in
series between the generator and converter.
IGBTs are connected in reverse direction each other both in
series and parallel connection The middle points of series are
connected to a capacitor
The MERS is inserted in series between ac power source
and load In the case when S1 and S3 are turned on, the current
flows in the positive direction When S1 and S3 are turned off,
the magnetic energy which has been stored in the inductance is
regenerated into the capacitor Next, the case when S2 and S4
are turned off, the capacitor discharges the energy to the load
and the current flows in the negative direction The phase of
current can be controlled by changing a switching phase angle
of MERS MERS itself generates voltage and compensates for
the inductance voltage unlike a conventional series capacitor,
so that another dc power supply is not needed Therefore,
by advancing the switching phase angle of S1 and S3 by 90
degrees, the inductive reactance voltage is compensated by the
capacitor voltage and the power factor of the circuit becomes
unity
III WIND POWER CONVERSION SYSTEMS WITHMERS
Figure 2 (a) is a conventional wind power system widely
used A variable speed synchronous generator with many poles
is connected to a grid through an ac-dc converter and a dc-ac
inverter Gearless is possible for this system, and it is more
efficient than a induction generator The disadvantage of this
system is that an output voltage of the synchronous generator
decreases with the increase of current because of synchronous
reactance Since the overload capacity of generator is small,
-H a
N s
N s
8 m e r s
terminal voltage.
N s1
8 m e r s
8 0
an instantaneous strong wind power energy cannot be taken out and it is not efficient
Figure 2 (b) is a wind power conversion system with MERS that we are proposing in this paper The MERS can improve a power factor regardless of the impedance and power frequency
of the load, and it generates voltage and compensates for the synchronous reactance voltage So, the output voltage of the generator increases, and it becomes possible to improve limited output power and efficiency of the wind power system Therefore, it is expected that the efficiency improvement of the power conversion system and the miniaturization of wind turbine generator can be expected Moreover, the MERS can
be used as an ac breaker
Figure 3 is an equivalent circuit of synchronous generator with MERS The equivalent circuit of synchronous generator is shown by series circuit of induced electromotive force ˙E and
synchronous reactancexsand armature winding resistancera When power factor of the load is assumed unity, the voltage
of the generator output terminal ˙V0 when MERS is not used
is shown by the following equation
˙V0 = ˙E − (ra + jxs) ˙I (1)
˙V0 decreases more than ˙E as the output current ˙I increases,
and the phase of ˙I is delayed than ˙E MERS can control the
phase of the current by an easy control, and correct the power
Trang 3I M S G
1 0 - k W
S G
I N V
M E R S x 3
3 7 k W
I n d u c t i o n
G a t e
D r i v e r x 1 2
A m p
P u l s e G e n e r a t o r / P h a s e S h i f t e r
R o t a r y
E n c o d e r
T o r q u e
M e t e r
A c t i v e
P o w e r
L o a d
G r i d
S 1
S 2
V C
C o n t r o l C i r c u i t
P r o t e c t i o n
C i r c u i t
V d c
Iu
1 5 - k W
P M S G o r
If
factor of induced electromotive force ˙E.
When MERS is inserted series, because ˙Vmers is equal to
xs˙I the output voltage ˙V is given by the following equation
Figure 4 shows the phase diagram of synchronous generator
with MERS The phase of induced electromotive force ˙E is
equal to the phase of output current ˙I The output voltage ˙V0is
increased compared with a system without MERS because the
leading current flows to the generator Therefore, the voltage
descent of output voltage ˙V is only the voltage of winding
resistance ra˙I, and the output voltage ˙V in a system with
MERS is increased compared with a system without MERS
In other words, the output voltage of a generator is recovered
by controlling the phase of output current and compensating
the synchronous reactance voltage by MERS
IV EXPERIMENTAL SETUP
Before comparing the MERS system with the conventional
back-to-back system, we will experimentally investigate
ef-fects of MERS in the newly proposed system shown in Figure
2 (b) We will compare the output of the newly proposed
system between with MERS and w/o MERS
A Wind turbine simulator
Figure 5 shows a schematic diagram of our small-scale
experimental system of wind power generation with a
syn-chronous generator This system consists of a M-G set and an
active power load The M-G set consists of a 1.5-kW interior
permanent magnet type synchronous generator (PMSG) or
a 1.0-kW dc-excitation type synchronous generator (DCSG)
coupled with a 3.7-kW squirrel cage induction motor The
rotation speed of the induction machine is controlled by an
inverter to simulate fluctuations of a wind
Fig 6 1.0-kW dc-exciting type synchronous generator and 3.7-kW induction motor (M-G set) The mechanical input power of the generator is measured with a torque meter.
Trang 4TABLE I
GENERATOR
TABLE II
GENERATOR
Overviews of the M-G set and MERS modules used in
this experimental system are shown in Figure 6 and Figure
7 Specifications of synchronous generators used for these
experiments are shown in table I and table II
MERSs are inserted in series between the generator and the
active power load The MERS is composed of four IGBTs
(1MB20D-060) and390 µF dc electrolytic capacitor.
B Gate signal control circuit
The phase of MERSs’ gate signals are made to advance
90 electrical degrees from an induced electromotive forceE.
However, the induced EMFE lags no load induced EMF E0
by power angleδ, the gate signal is shifted by δ as shown in
Figure 8 The phase angle of the rotor is detected by a rotary
encoder installed in the generator And gate signals are given to
each IGBT element through amplifiers and gate drive circuits
Because the switching frequency of MERS is the same as the
armature voltage frequency, switching loss of IGBT is so small
that it can be disregarded
S 1 , S 3
S 2 , S 4
O N
O F F
O N
O F F
−200 0 200
−10 0 10
time (ms)
−200 0 200
−10 0
10
0 5 10 15
time (ms)
VCu
of MERS (u-phase).
A Permanent magnet type synchronous generator
The measured voltage and current waveforms are shown
in Figure 9 Though the same output current Iu of 5.7 A is observed for with and w/o MERS, the higher dc output voltage
Vdc of 180.3 V is obtained in the system with MERS, while
Vdc is 143.0 V in the system w/o MERS The average value
of capacitor voltage VC is 51.3 V The voltage drop across the synchronous reactance is compensated by this capacitor voltage and the output voltage Vdc is increased
Trang 50 4 8
0
100
200
Ohase current Iu (A)
Vdc
with MERS
w/o MERS
(a) Dc output voltage
0
1
2
Output current Iu (A)
Pout
with MERS
w/o MERS
(b) Dc output power
Figure 10 (a) shows the comparison of the dc output voltage
of PMSG with MERS and without MERS The output voltage
Vdc decreased with an increase of output current Iu in both
system However, the voltage reduction by synchronous
reac-tance was recovered by MERS and output voltage increased in
the system with MERS It becomes only a voltage drop by the
resistance, and the characteristic of the synchronous generator
is the same as a direct current generator
Figure 10 (b) shows experimental results of dc output power
as a function of output current Iu The maximum output is
1.2 kW in the system without MERS, while the maximum
output is 2 kW or more in the system with MERS These
data indicate that an instantaneous strong wind power can be
caught by a generator with the MERS system, and thereby the
generator with a small ratings can be used for wind power
0 1 2 3
Output Power (p.u.)
with MERS w/o MERS
0 10
Output Power (p.u.)
with MERS w/o MERS
MERS and without MERS.
generation Moreover, even when the output power is kept constant, the output current can be decreased in the MERS system because the output voltage increases As a result, a copper loss and an amount of produced heat of the generator can be suppressed, and a miniaturization of the generator can
be expected by applying the MERS to the power conversion system
B Dc excitation type synchronous generator
Figure 11 shows the comparison of the excitation power of experimental DCSG system with MERS and without MERS The excitation power is normalized by the no-load excitation power The necessary excitation power of the generation system with MERS to maintain the dc output voltage of 270 V was smaller than that of without MERS In the case of the synchronous reactance of 0.73 p.u., the necessary excitation
Trang 6M E R S
S G
0 2 m H ( 0 1 p u )
1 6 8
6 9 0 V
D C V
6 9 0 V , 6 2 7 A
X s = 0 4 4
F i l t e r
G r i d
T r a n s f o r m e r
S G
W i n d T u r b i n e
0 2 m H ( 0 1 p u )
1 6 8
6 9 0 V
0 2 m H ( 0 1 p u )
1 6 8
6 9 0 V
D C 1 0 0 0 V
6 9 0 V , 6 2 7 A
X s = 0 4 4
F i l t e r
P W M
G r i d
F i l t e r
( a ) P W M c o n v e r t e r s y s t e m
( b ) M E R S c o n v e r t e r s y s t e m
with MERS Input filter is not required in the MERS system.
power with MERS is about 65% of that without MERS at the
output power of 1 p.u In the case of 2.74 p.u., it is reduced
to only 21% The effect of compensating the reactance voltage
by MERS is extremely large in the latter case due to the very
high reactance voltage
VI CONVERTER LOSS
The experimental results indicate that the newly proposed
power conversion system with MERS has a great potential
to improve the performance compared with the conventional
system The remarkable advantage of this new system is a big
improvement in the output voltage and the overload output
capacity of the generator The latter advantage suggests that
the wind turbine generator can be designed much smaller than
in the conventional system, which may be much cost effective
Figure 12 shows 750-kW wind power conversion system
with PWM converter and with MERS An input filter is not
required in the system with MERS because of low switching
frequency To compare the loss of these power conversion
systems, suppose that the line voltage is 690 V and rated
output power of wind turbine generator is 750 kW, and the
output current and voltage of the generator was calculated
from the experiment results Figure 13 shows the result of
comparing loss of power conversion system with MERS, and
PWM converter The rated output power of the wind turbine
used for this calculation is 750 kW The conduction losses of
IGBTs increase in the MERS system However, turn-on and
turn-off switching losses of MERS are so small that it can be
disregarded because the switching frequency is the same as
the generator Moreover, there is no loss of the filter because
a filter on the converter input side is unnecessary The total
loss of the power conversion system with MERS is 39.7 kW,
while that of PWM converter is 54.2 kW, it is reduced to 73%
3 9 7 k W
5 4 2 k W
M E R S C o n v e r t e r ( 5 0 H z ) P W M C o n v e r t e r( 5 k H z )
I G B T O N
F W D R e v e r s e
R e c o v e r y
D i o d e
R e c t i f i e r
F i l t e r
F i l t e r
I G B T O F F
F W D O N
P W M
I n v e r t e r
C o n d u c t i o n
l o s s o f I G B T s
750 kW.
A low speed switching IGBT can be used for the MERS system because the switching frequency of MERS is the same as a frequency of the generator Further improvement
in efficiency of power conversion system with MERS would
be realized if the combination ofVCEand switching frequency were optimized
VII CONCLUSION
This paper presents the experimental results to improve the output power and efficiency of a wind turbine system with a synchronous generator by applying magnetic energy recovery switch (MERS) to the power conversion system The wind power conversion system with MERS compensates the voltage drop across a synchronous reactance in a synchronous generator The MERS system improves the overload capacity
of the generator and the necessary excitation power is greatly reduced Also, the loss of converter is reduced to 70% or less and a miniaturization of power conversion system becomes possible The experimental results indicate a great potential of the newly developed power conversion system using MERS
to make the actual wind turbine system with a synchronous generator more compact and to improve the system efficiency compared with the conventional systems
[1] K Shimada et al: “Bi-directional current switch with snubber regen-eration using P-MOSFETs,” in Proc International Power Electronics Conference, Apr 2000, no 3, pp 1519–1524.
[2] T Takaku et al: “Automatic Power Factor Correction Using Magnetic Energy Recovery Switch,” in Proc I E E Japan, vol 125-D, no 4,
2005 (in Japanese).
[3] T Takaku et al: “Power Supply for Pulsed Magnets with Magnetic Energy Recovery Current Switch,” in IEEE Transactions on Applied Superconductivity, vol 14, no 2, pp 1794–1797, 2004.